U.S. Data Sources

ArcGIS.comData downloads, WMS links, and/or access via ArcGIS Online and ArcGIS layer files.
The Esri Maps and Data Group, as well as other groups "owned by Esri" are highly recommended for their various basemap layers. Predominantly US-based data, but also some from international sources.

USGS National Map
Download site for multiple data layers, including topography, hydrography, transportation, geographic names, boundaries, land cover, and more. The Small-Scale collection is good for contextual (overview) or large extent maps as most of the data were compiled at a scale of 1:2,000,000.